Embodiments of the present invention relate generally to photovoltaic (PV) power systems and, more particularly, to a PV power system that implements active diagnostics and provides for ground fault detection in grounded conductors of individual PV strings.
PV power systems are power systems that employ a plurality of solar modules to convert sunlight into electricity. PV systems include multiple components, including photovoltaic modules, mechanical and electrical connections and mountings, and means of regulating or modifying the electrical output. One common arrangement in PV systems is for several PV modules to be connected in series to form a PV string, with multiple PV strings in a PV system then being combined in parallel to aggregate the current in a PV array. The PV modules generate direct current (DC) power, with the level of DC current being dependent on solar irradiation and the level of DC voltage dependent on temperature. When alternating current (AC) power is desired, an inverter is used to convert the DC energy from the array into AC energy, such as AC energy suitable for transfer to a power grid.
PV power systems also include a balance-of-system comprising DC switching and protection devices, combiner boxes, circuit breakers, disconnect switches, and contactors. Combiner boxes aggregate the DC power from the PV strings and provide a parallel connection point (i.e., a common bus) for the PV strings, with the combiner box providing overcurrent protection and isolation means (i.e., disconnect). Combiner boxes are either source combiners or array combiners, with source combiners being located closer to the PV strings and array combiners—or re-combiners—aggregating outputs from several source combiners into a single circuit.
It is recognized that there are several system design and component challenges associated with the operation of PV power systems, including system monitoring and ground fault detection and protection. With respect to system monitoring, such monitoring in existing PV power systems is based on current measurements performed by current sensors in the PV systems. The current sensors might be located inside the combiner box, with each string having a current sensor or two or more strings sharing a current sensor, the current sensors might be located inside the recombiner box, with each input of the recombiner box having a current sensor, or the current sensors might be located inside the PV inverter. The current measurements from current sensors are sampled periodically (e.g., sampling rate of 15 minutes) and this current data is then to central service for analyzing by an algorithm. The algorithm identifies degraded strings or degraded groups of strings (depending on current sensor locations) by data-mining current measurement with values lower than the average/typical current.
The limitation of the state-of-art PV system monitoring is that it can only identify degraded string or degraded group of strings, but it cannot diagnose the problem/reasons that causes the degradation. This is because at any moment in time with a central inverter, there is a single Maximum Power Point (MPP) voltage for the array.
With respect to ground fault detection and protection, such ground fault detection and protection in existing PV power systems is typically ground fault detection, shutdown, and alarming at the inverter for ground faults within the PV array. However, the state-of-the-art ground fault detection only can detect ground faults in ungrounded conductors, it cannot detect ground faults in grounded conductors.
It would therefore be desirable to provide a PV power system and method of monitoring thereof that provides for active diagnostics in the PV system and diagnosis of the reasons that causes degradation of PV strings (or groups of PV strings) in the PV system. It would further be desired to for such a PV system and method of monitoring thereof to provide the capability to detect ground faults in a grounded current-carrying conductor.